1. Field of the Invention
The invention refers to implantable medical devices (IMDs) providing means for electrical stimulation of heart chambers and a high rate protection preventing excessive stimulation rates. The invention relates in particular to implantable cardiac pacemaker or an implantable cardioverter/defibrillator (ICD) for bi-ventricular stimulation.
2. Description of the Related Art
Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. A strong enough stimulation pulse outside a heart chamber's refractory period leads to an excitation of the myocardium of that heart chamber, which in turn is followed by a contraction of the respective heart chamber.
Depending on the disorder to be treated, such a heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse.
Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively.
In biventricular pacemakers both ventricles of a heart are paced.
Biventricular pacemakers are used for a cardiac re-synchronization therapy that shall synchronize right ventricular contraction and left ventricular contraction to improve the output of a heart exhibiting a cardiovascular disease. In particular, cardiac synchronization therapy is used to treat heart failure in patients with wide QRS complex that results from a delayed excitation of the left heart side. It is believed, that a main contributor to heart failure (the heart's inability to generate enough cardiac output) is an asynchronous mechanical contraction of the left and right side of the heart.
In a heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. If the myocardium is fully depolarized, it is unsusceptible for further excitation and thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber is expanding again. In a typical electrogram (EGM), depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”.
After a pace event, the cardiac tissue in the paced chamber is refractive for at least 125 ms. This means that another pace during this period will not have any effect. The tissue then repolarises until it reaches the resting state, typically no later than 270 ms after the pace. The phase between the time when the tissue is refractive and the time when it reaches the resting state is known as the vulnerable period that approximately coincides with a T-wave. A second pace event during this time may cause a potentially fatal tachycardia.
Cardiac rhythm management (CRM) devices are designed to avoid pacing in the vulnerable period. However, a fault could result in this undesired behaviour. As an example of such a fault, the crystal oscillator circuit could lock on a harmonic of the crystal. Another cause for pacing in the vulnerable period could be a modification of the CRM device configuration due to ambient radiation (soft error).
This issue is recognized to the extent that a European Standard for pacemakers (EN 45502-2-1:2003, section 16.4) mandates the following:
The design of the implantable pulse generator shall include a feature to limit the pulse rate in the event of a fault within the device (run-away protection).
Dual chamber devices implement a high-rate protection (HRP) circuit on each of the two chambers: atrium and ventricle. These HRP circuits use an independent time base and pre-vent consecutive pace events from occurring closer than a chosen interval, typically 270 ms.